7,488 research outputs found
Theory of Stellar Oscillations
In recent years, astronomers have witnessed major progresses in the field of
stellar physics. This was made possible thanks to the combination of a solid
theoretical understanding of the phenomena of stellar pulsations and the
availability of a tremendous amount of exquisite space-based asteroseismic
data. In this context, this chapter reviews the basic theory of stellar
pulsations, considering small, adiabatic perturbations to a static, spherically
symmetric equilibrium. It starts with a brief discussion of the solar
oscillation spectrum, followed by the setting of the theoretical problem,
including the presentation of the equations of hydrodynamics, their
perturbation, and a discussion of the functional form of the solutions.
Emphasis is put on the physical properties of the different types of modes, in
particular acoustic (p-) and gravity (g-) modes and their propagation cavities.
The surface (f-) mode solutions are also discussed. While not attempting to be
comprehensive, it is hoped that the summary presented in this chapter addresses
the most important theoretical aspects that are required for a solid start in
stellar pulsations research.Comment: Lecture presented at the IVth Azores International Advanced School in
Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars
and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta,
Azores Islands, Portugal in July 201
Simulation of quantum random walks using interference of classical field
We suggest a theoretical scheme for the simulation of quantum random walks on
a line using beam splitters, phase shifters and photodetectors. Our model
enables us to simulate a quantum random walk with use of the wave nature of
classical light fields. Furthermore, the proposed set-up allows the analysis of
the effects of decoherence. The transition from a pure mean photon-number
distribution to a classical one is studied varying the decoherence parameters.Comment: extensively revised version; title changed; to appear on Phys. Rev.
Second harmonics and compensation effect in ceramic superconductors
A three-dimensional lattice of the Josephson junctions with a finite
self-conductance is employed to model the ceramic superconductors. The
nonlinear ac susceptibility and the compensation effect are studied by Monte
Carlo simulations in this model. The compensation effect is shown to be due to
the existence of the chiral glass phase. We demonstrate, in agreement with
experiments, that this effect may be present in the ceramic superconductors
which show the paramagnetic Meissner effect.Comment: 6 pages, latex, 4 figures, Phys. Rev. B (accepted
Picturing classical and quantum Bayesian inference
We introduce a graphical framework for Bayesian inference that is
sufficiently general to accommodate not just the standard case but also recent
proposals for a theory of quantum Bayesian inference wherein one considers
density operators rather than probability distributions as representative of
degrees of belief. The diagrammatic framework is stated in the graphical
language of symmetric monoidal categories and of compact structures and
Frobenius structures therein, in which Bayesian inversion boils down to
transposition with respect to an appropriate compact structure. We characterize
classical Bayesian inference in terms of a graphical property and demonstrate
that our approach eliminates some purely conventional elements that appear in
common representations thereof, such as whether degrees of belief are
represented by probabilities or entropic quantities. We also introduce a
quantum-like calculus wherein the Frobenius structure is noncommutative and
show that it can accommodate Leifer's calculus of `conditional density
operators'. The notion of conditional independence is also generalized to our
graphical setting and we make some preliminary connections to the theory of
Bayesian networks. Finally, we demonstrate how to construct a graphical
Bayesian calculus within any dagger compact category.Comment: 38 pages, lots of picture
Strong rejuvenation in a chiral-glass superconductor
The glassy paramagnetic Meissner phase of a BiSrCaCuO
superconductor ( = 8.18) is investigated by squid magnetometry, using
``dc-memory'' experiments employed earlier to study spin glasses. The
temperature dependence of the zero-field-cooled and thermo-remanent
magnetization is recorded on re-heating after specific cooling protocols, in
which single or multiple halts are performed at constant temperatures. The
'spin' states equilibrated during the halts are retrieved on re-heating. The
observed memory and rejuvenation effects are similar to those observed in
Heisenberg-like spin glasses.Comment: REVTeX 4 style; 5 pages, 5 figure
Louse (Insecta : Phthiraptera) mitochondrial 12S rRNA secondary structure is highly variable
Lice are ectoparasitic insects hosted by birds and mammals. Mitochondrial 12S rRNA sequences obtained from lice show considerable length variation and are very difficult to align. We show that the louse 12S rRNA domain III secondary structure displays considerable variation compared to other insects, in both the shape and number of stems and loops. Phylogenetic trees constructed from tree edit distances between louse 12S rRNA structures do not closely resemble trees constructed from sequence data, suggesting that at least some of this structural variation has arisen independently in different louse lineages. Taken together with previous work on mitochondrial gene order and elevated rates of substitution in louse mitochondrial sequences, the structural variation in louse 12S rRNA confirms the highly distinctive nature of molecular evolution in these insects
Quantum Monte Carlo study of a nonmagnetic impurity in the two-dimensional Hubbard model
In order to investigate the effects of nonmagnetic impurities in strongly
correlated systems, Quantum Monte Carlo (QMC) simulations have been carried out
for the doped two-dimensional Hubbard model with one nonmagnetic impurity.
Using a bare impurity potential which is onsite and attractive, magnetic and
single-particle properties have been calculated. The QMC results show that
giant oscillations develop in the Knight shift response around the impurity
site due to the short-range antiferromagnetic correlations. These results are
useful for interpreting the NMR data on Li and Zn substituted layered cuprates.Comment: 10 pages, 7 figure
Stationary Dyonic Regular and Black Hole Solutions
We consider globally regular and black hole solutions in SU(2)
Einstein-Yang-Mills-Higgs theory, coupled to a dilaton field. The basic
solutions represent magnetic monopoles, monopole-antimonopole systems or black
holes with monopole or dipole hair. When the globally regular solutions carry
additionally electric charge, an angular momentum density results, except in
the simplest spherically symmetric case. We evaluate the global charges of the
solutions and their effective action, and analyze their dependence on the
gravitational coupling strength. We show, that in the presence of a dilaton
field, the black hole solutions satisfy a generalized Smarr type mass formula.Comment: 23 pages, 4 figure
Adiabatic response for Lindblad dynamics
We study the adiabatic response of open systems governed by Lindblad
evolutions. In such systems, there is an ambiguity in the assignment of
observables to fluxes (rates) such as velocities and currents. For the
appropriate notion of flux, the formulas for the transport coefficients are
simple and explicit and are governed by the parallel transport on the manifold
of instantaneous stationary states. Among our results we show that the response
coefficients of open systems, whose stationary states are projections, is given
by the adiabatic curvature.Comment: 33 pages, 4 figures, accepted versio
Observability and nonlinear filtering
This paper develops a connection between the asymptotic stability of
nonlinear filters and a notion of observability. We consider a general class of
hidden Markov models in continuous time with compact signal state space, and
call such a model observable if no two initial measures of the signal process
give rise to the same law of the observation process. We demonstrate that
observability implies stability of the filter, i.e., the filtered estimates
become insensitive to the initial measure at large times. For the special case
where the signal is a finite-state Markov process and the observations are of
the white noise type, a complete (necessary and sufficient) characterization of
filter stability is obtained in terms of a slightly weaker detectability
condition. In addition to observability, the role of controllability in filter
stability is explored. Finally, the results are partially extended to
non-compact signal state spaces
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